1. Field of the Invention
The present invention relates to a digital radio communication system, and more specifically to such a system which is well-suited for mobile radio communications and which utilizes linear quadrature modulated waves to transmit a binary digital signal.
2. Description of the Prior Art
Recent developments in digital techniques, particularly digital modulation and demodulation techniques, have rendered digital communication systems a reality. However, in order to practically realize such a system in which economical transmission of information is possible while maintaining confidentiality and high quality, it is necessary to meet the following various technical requirements:
(1) the transmission spectrum should be sufficiently narrow to enable effective use within the limited radio frequencies;
(2) the transmitter should have a low power consumption; and
(3) the receiver should be highly compact, simple and feature excellent error correction characteristics for level variations in the received signal.
One known technique directed to meeting the above-mentioned requirements, is disclosed on pages 534 to 542 of a paper presented by Frank de Jager et al in the IEEE Transactions on Communications, Vol. COM-26 No. 5, entitled "Tamed Frequency Modulation, A Novel Method to Achieve Spectrum Economy in Digital Transmission". This Tamed Frequency Modulation (TFM) is a digital non-linear type for transmitting digital information using a modulated carrier signal with a constant envelope. The TFM has resulted from efforts to improve the transmission spectrum in an MSK (Minimum Shift Keying) system which have been developed in the field of satellite communication. This prior art is particularly suitable for use in mobile radio communication systems which require effective suppression of interchannel interference and low power consumption.
The TFM method can utilize a saturated type class C power amplifier in view of the fact that the modulated carrier signal has a constant envelope. Although such a power amplifier features low power consumption, the transmission spectrum of the TFM method depends on the frequency bandwidth of baseband signal and modulation index, and hence is inherently wide relative to a linear modulation method. In order to narrow the transmission spectrum with the above-mentioned prior art, special circuitry is needed for partial response wave-shaping of the modulation signals. This means that, in a mobile communication system wherein the channel spacing is 25 kHz (for example), the upper maximum limit of a signal transmission rate is restricted to 16K bits/second. Further, in the case where a receiver utilizes a coherent detection, as discussed in the above-mentioned prior art, the receiver becomes complex due to the recovery of a coherent carrier signal, thereby lowering the error rate performance of received signals due to carrier signal phase slipping caused by fast fading.
On the other hand, in a digital linear modulation system, the spectrum of the modulated carrier signal can be narrowed by utilizing the baseband signals which are derived from a low-pass filter as the modulation signals. The modulated carrier signal in this instance, however, is not constant in its envelope and hence is apt to reduce the power efficiency in that a class A amplifier is required. Further, the receiver installed in such a system requires an automatic gain controller to compensate for received signal level variations. In particular mobile radio communication systems are subject to level variations exceeding 100 dB due to the fading and transmission path variations, so that the receivers thereof become undesirably complex and bulky.